Heat-dissipating sheet having high thermal conductivity and its production method

ABSTRACT

A heat-dissipating sheet having a density of 2.0 g/cm 3  or more and an in-plane thermal conductivity of 580 W/mK or more, which comprises carbon black uniformly dispersed among fine graphite particles, a mass ratio of fine graphite particles to carbon black being 75/25 to 95/5, and the carbon black being composed of channel black and ketjen black and/or acetylene black is produced by applying a dispersion of fine graphite particles, carbon black and an organic binder in an organic solvent to a surface of a die, drying it; burning the resultant resin-containing composite sheet to remove the organic binder; and then pressing the resultant composite sheet of fine graphite particles and carbon black for densification.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending application Ser. No.14/535,979 filed on Nov. 7, 2014, which claims priority under 35 U.S.C.§ 119(a) to Patent Application No. 2014-95419 filed in Japan on May 2,2014. All of the above applications are hereby expressly incorporated byreference into the present application.

FIELD OF THE INVENTION

The present invention relates to a heat-dissipating sheet having highthermal conductivity for efficiently dissipating heat generated fromelectronic parts, etc. in small electronic appliances such as note-typepersonal computers, smartphones, mobile phones, etc., and its productionmethod.

BACKGROUND OF THE INVENTION

In small electronic appliances such as note-type personal computers,smartphones, mobile phones, etc., which have been provided withincreasingly higher performance and more functions, electronic devicessuch as microprocessors, imaging chips, memories, etc. should be mounteddensely. Accordingly, to prevent malfunction due to heat generated bythem, the dissipation of heat generated from such electronic devices hasbecome increasingly important.

As a heat-dissipating sheet for electronic devices, JP 2006-306068 Adiscloses a heat-conductive sheet comprising at least a graphite filmand an adhesive resin composition, which is a reaction-curable vinylpolymer. The graphite film is (a) expanded graphite formed by anexpanding method, or (b) obtained by heat-treating a polyimide film,etc., at a temperature of 2400° C. or higher. The expanded graphite filmis obtained by immersing graphite in acid such as sulfuric acid, etc. toform a graphite interlayer compound, heat-treating the graphiteinterlayer compound to foam it, thereby separating graphite layers,washing the resultant graphite powder to remove acid, and rolling theresultant thin-film graphite powder. However, the expanded graphite filmhas insufficient strength. Also, the graphite film obtained by the heattreatment of a polyimide film, etc. is disadvantageously expensivedespite high heat dissipation.

JP 2012-211259 A discloses a heat-conductive sheet comprising graphitepieces, which comprise pluralities of first graphite pieces obtained bythinly cutting a thermally decomposed graphite sheet, and secondgraphite pieces smaller than the widths of the first graphite pieces, atleast the first graphite pieces connecting both surfaces of theheat-conductive sheet. This heat-conductive sheet is obtained, forexample, by blending the first and second graphite pieces with a mixtureof an acrylic polymer and a solvent, and extruding the resultant blend.However, the extruded heat-conductive sheet does not have sufficientheat dissipation, because of a high volume fraction of the resin.

JP 2006-86271 A discloses a heat-dissipating sheet as thick as 50-150 μmcomprising graphite bonded by an organic binder having a glasstransition temperature of −50° C. to +50° C., such as an amorphouscopolyester, a mass ratio of graphite/binder resin being 66.7/33.3 to95/5. This heat-dissipating sheet is produced by applying a slurry ofgraphite and an organic binder in an organic solvent to aparting-agent-coated film on the side of a parting layer, drying theslurry by hot air to remove the organic solvent, and then pressing it,for example, at 30 kg/cm². JP 2006-86271 A describes that the pressingof a graphite/organic binder sheet improves its thermal conductivity.However, because this heat-dissipating sheet contains an organic binder,it does not sufficiently exhibit high thermal conductivity inherent ingraphite.

JP 11-1621 A discloses a high-thermal-conductivity, solid compositematerial for a heat dissipater comprising highly oriented graphiteflakes and a binder polymer polymerized under pressure. This solidcomposite material is produced by mixing graphite flakes with athermosetting monomer such as an epoxy resin to prepare a compositioncomprising at least 40% by volume of graphite, and polymerizing themonomer while compressing the composition under sufficient pressure toalign graphite substantially in parallel. However, because this solidcomposite material comprises an epoxy resin, it does not havesufficiently high thermal conductivity.

JP 2012-136575 A discloses a conductive, heat-dissipating sheetcomprising organic particles made of polyamides, acrylic resins, etc.and having an average particle size of about 0.1-100 μm, conductiveinorganic fillers having an average particle size of about 10 nm toabout 10 μm, and a cured resin such as an epoxy resin, etc., organicparticles/inorganic fillers being 1000/1 to 10/1, and the percentage ofinorganic fillers being 5-30% by weight based on the total amount. JP2012-136575 A illustrates graphite, coke, carbon black, etc. asinorganic fillers, though only carbon black is used in Examples. Inaddition, this conductive heat-dissipating sheet does not havesufficient heat dissipation, because it contains the cured resin.

As described above, conventional heat-dissipating sheets containinggraphite or carbon black do not have sufficient heat dissipation becausethey also contain binder resins. Increase in the percentage of graphiteor carbon black results in lower sheet strength despite improved thermalconductivity, particularly causing the problem of easy detachment ofgraphite or carbon black from the heat-dissipating sheet. Accordingly,inexpensive heat-dissipating sheets having uniform, high heatdissipation as well as mechanical properties necessary for handling aredesired.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide aninexpensive heat-dissipating sheet having high heat dissipation as wellas mechanical properties necessary for handling, and its productionmethod.

DISCLOSURE OF THE INVENTION

As a result of intensive research in view of the above object, theinventor has found that (a) a heat-dissipating sheet comprising a smallamount of carbon black uniformly dispersed among fine graphite particleshas high thermal conductivity, as well as sufficient mechanicalproperties for handling, with substantially no detachment of finegraphite particles and carbon black; that (b) such heat-dissipatingsheet is obtained by forming a sheet comprising fine graphite particlesand carbon black dispersed in a small amount of an organic binder,burning the sheet to remove the organic binder, and pressing theresultant composite sheet of graphite and carbon black fordensification; and that (c) when a mixture of channel black and ketjenblack and/or acetylene black is used as carbon black, the resultantheat-dissipating sheet has improved thermal conductivity and mechanicalproperties. The present invention has been completed based on suchfindings.

Thus, the heat-dissipating sheet of the present invention has astructure in which carbon black is uniformly dispersed among finegraphite particles,

a mass ratio of fine graphite particles to carbon black being 75/25 to95/5;

the carbon black being composed of channel black and ketjen black and/oracetylene black; and

the heat-dissipating sheet having a density of 2.0 g/cm³ or more and anin-plane thermal conductivity of 580 W/mK or more.

A mass ratio of channel black to ketjen black and/or acetylene black ispreferably 4/1 to ⅓.

The heat-dissipating sheet preferably has thickness of 25-150 μm.

The fine graphite particles preferably have an average diameter of 3-150μm and average thickness of 200 nm or more.

The carbon black preferably has an average primary particle size of20-200 nm.

The heat-dissipating sheet is preferably coated with insulating resinlayers or insulating plastic films.

The method of the present invention for producing the aboveheat-dissipating sheet comprises the steps of (1) preparing a dispersionof fine graphite particles, carbon black and an organic binder in anorganic solvent, a mass ratio of the fine graphite particles to thecarbon black being 75/25 to 95/5, and the carbon black being composed ofchannel black and ketjen black and/or acetylene black; (2) casting thedispersion into a cavity of a lower die plate and then drying it to forma resin-containing composite sheet comprising the fine graphiteparticles, the carbon black and the organic binder; (3) burning theresin-containing composite sheet to remove the organic binder to form acomposite sheet of fine graphite particles and carbon black; and (4)pressing the lower die plate combined with an upper die plate to densifythe composite sheet of fine graphite particles and carbon black.

The dispersion preferably comprises 5-25% by mass in total of finegraphite particles and carbon black, and 0.5-2.5% by mass of the organicbinder.

A mass ratio of the organic binder to the total amount of the finegraphite particles and the carbon black is preferably 0.01-0.5.

The organic binder is preferably an acrylic resin, a polystyrene resinor polyvinyl alcohol.

The organic solvent is preferably at least one selected from the groupconsisting of ketones, aromatic hydrocarbons and alcohols.

The burning step is preferably conducted at a temperature of 550-700° C.

Cooling to room temperature after burning is preferably graduallyconducted over 1 hour or more.

The pressing step is preferably conducted, after the composite sheet offine graphite particles and carbon black is cooled to a temperatureequal to or lower than the freezing point of water.

The pressing step is preferably conducted at pressure of 20 MPa or more.

The resin-containing composite sheet formed in the lower die platecavity is preferably burned without being peeled from the lower dieplate, and then pressed with the lower die plate combined with the upperdie plate.

The pressing step is preferably conducted at a temperature in a range ofroom temperature to 200° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing the structure of aheat-dissipating sheet composed of fine graphite particles and carbonblack.

FIG. 2 is a cross-sectional view showing a method for determining theparticle size of a fine graphite particle.

FIG. 3 is a perspective view showing an example of planar dieapparatuses for forming a resin-containing composite sheet from adispersion of fine graphite particles, carbon black and an organicbinder in an organic solvent, and burning it.

FIG. 4 is a perspective view schematically showing a dispersion castinto a cavity of a lower die plate shown in FIG. 3.

FIG. 5(a) is an exploded perspective view showing another example oflower die plates in a planar die apparatus for forming aresin-containing composite sheet from a dispersion of fine graphiteparticles, carbon black and an organic binder in an organic solvent, andburning it.

FIG. 5(b) is an exploded perspective view showing an example ofcombinations of the lower die plate of FIG. 5(a) with an upper dieplate.

FIG. 6 is a perspective view schematically showing a dispersion cast inthe lower die plate of FIG. 5(b).

FIG. 7(a) is an exploded plan view showing a further example of planardie apparatuses usable in the present invention.

FIG. 7(b) is a cross-sectional view taken along the line A-A in FIG.7(a).

FIG. 7(c) is a cross-sectional view taken along the line B-B in FIG.7(a).

FIG. 7(d) is a cross-sectional view taken along the line C-C in FIG.7(a).

FIG. 7(e) is an exploded cross-sectional view showing a combination ofan upper die plate, an intermediate die plate and a lower die plateconstituting the planar die apparatus of FIG. 7(a).

FIG. 7(f) is a cross-sectional view showing a cavity obtained bycombining a lower die plate and an intermediate die plate in the planardie apparatus of FIG. 7(a).

FIG. 8 is a cross-sectional view showing a dispersion cast into thecavity of FIG. 7(f), and made uniform in thickness by removing anexcessive dispersion by a doctor blade method.

FIG. 9 is a perspective view showing a resin-containing composite sheetobtained by drying a dispersion of fine graphite particles and carbonblack in an organic solvent cast into the cavity of the lower die plateof FIG. 4, and a composite sheet of fine graphite particles and carbonblack obtained by burning the resin-containing composite sheet.

FIG. 10 is a perspective view showing a combination of a lower die platehaving a composite sheet of fine graphite particles and carbon black inits cavity, with an upper die plate.

FIG. 11 is a partially cross-sectional side view showing theroll-pressing of a composite sheet of fine graphite particles and carbonblack in a cavity of a planar die apparatus.

FIG. 12 is a perspective view showing the peeling of a heat-dissipatingsheet obtained by pressing from a lower die plate.

FIG. 13 is a schematic cross-sectional view showing a heat dissipationtest apparatus of a heat-dissipating sheet.

FIG. 14 is an exploded view of FIG. 13.

FIG. 15 is a plan view showing temperature-measuring points on aheat-dissipating sheet test piece set in a heat dissipation testapparatus.

FIG. 16 is a graph showing the relation between the ratio of channelblack in carbon black and in-plane thermal conductivity, in theheat-dissipating sheet of Example 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be explained in detailbelow referring to the attached drawings. Explanations of eachembodiment are applicable to other embodiments unless otherwisementioned. Explanations below are not restrictive, but variousmodifications may be made within the scope of the present invention.

[1] Heat-Dissipating Sheet

As shown in FIG. 1, the heat-dissipating sheet 1 of the presentinvention is substantially composed only of fine graphite particles 2,and carbon black 3 uniformly dispersed among fine graphite particles 2.The term “substantially composed only of” means that theheat-dissipating sheet 1 of the present invention obtained by burningand pressing a composite sheet of fine graphite particles, carbon blackand an organic binder does not contain other components than finegraphite particles and carbon black except for burned residues. Thoughgaps between fine graphite particles 2 and carbon black 3 areexaggerated in FIG. 1 for clarity, the fine graphite particles 2 and thecarbon black 3 are actually bonded closely with substantially no gaps.

(1) Fine Graphite Particles

A fine graphene particle has a flake- or plate-like, multi-layerstructure, in which benzene rings are two-dimensionally connected.Because the fine graphene particle has a hexagonal lattice structure,each carbon atom is bonded to three carbon atoms, one of four peripheralelectrons used for chemical bonding being in a free state (freeelectron). Because free electrons can move along the crystal lattice,fine graphene particles have high thermal conductivity.

Because a fine graphene particle has a flake- or plate-like shape, itssize is represented by the diameter of its planar surface. Because aflake-like, fine graphene particle 2 has a planar contour of anirregular shape as shown in FIG. 2, the size (diameter) of each finegraphene particle 2 is defined as a diameter d of a circle having thesame area S. Because the size of each fine graphene particle 2 isexpressed by a diameter d and a thickness t, the average diameter offine graphene particles 2 used is expressed by (Σd)/n, wherein nrepresents the number of fine graphene particles 2 measured, and theaverage thickness of fine graphene particles 2 is expressed by (Σt)/n.The diameters d and thickness t of fine graphene particles 2 can bedetermined by the image treatment of photomicrographs of fine grapheneparticles 2.

The average diameter of fine graphene particles 2 used in the presentinvention is preferably in a range of 3-150 μm. When the averagediameter of fine graphene particles 2 is less than 3 μm, bonded carbonatoms are not sufficiently long, providing a heat-dissipating sheet 1with too small thermal conductivity. On the other hand, fine grapheneparticles 2 having an average diameter of more than 150 μm would makespray coating difficult. The average diameter of fine graphene particles2 is more preferably 5-100 μm, further preferably 5-50 μm, mostpreferably 10-30 μm. The average thickness of fine graphene particles 2is preferably 200 nm or more, more preferably 200 nm to 5 μm, mostpreferably 200 nm to 1 μm.

(2) Carbon Black

Carbon black 3 used in the present invention is constituted by channelblack and ketjen black and/or acetylene black. The mass ratio of channelblack to ketjen black and/or acetylene black is preferably 4/1 to ⅓.Within this mass ratio range, the heat-dissipating sheet 1 has highthermal conductivity and good mechanical properties (tensile strength,bendability and cuttability). The more preferred mass ratio of channelblack to ketjen black and/or acetylene black is 3/1 to ½. Incidentally,the mass ratio of ketjen black to acetylene black may be from 0% to100%.

Carbon black 3 preferably has an average primary particle size of 20-200nm. With carbon black 3 having an average primary particle size of lessthan 20 nm, agglomeration is likely to occur, making difficult theuniform dispersion of carbon black 3 among fine graphite particles 2.Carbon black 3 having an average primary particle size of more than 200nm is too large to be uniformly dispersed among fine graphite particles2. The average primary particle size of carbon black 3 is morepreferably 30-100 nm, most preferably 30-80 nm.

(3) Mass Ratio

The mass ratio of fine graphite particles to carbon black is 75/25 to95/5. Within the above mass ratio range of fine graphite particles tocarbon black, a heat-dissipating sheet having as high in-plane thermalconductivity as 580 W/mK or more and sufficient mechanical properties(tensile strength, bendability and cuttability) for handling can beobtained. When fine graphite particles is more than 95% by mass (carbonblack is less than 5% by mass), their total amount being 100% by mass, asufficient effect of adding carbon black cannot be obtained. On theother hand, when fine graphite particles are less than 75% by mass(carbon black is more than 25% by mass), a heat-dissipating sheet havingan in-plane thermal conductivity of 580 W/mK or more cannot be obtained.The mass ratio of fine graphite particles to carbon black is preferably80/20 to 95/5, more preferably 82.5/17.5 to 90/10.

(4) Thickness

To secure sufficient cooling power, the heat-dissipating sheet ispreferably as thick as 25-150 μm. When it is thinner than 25 μm, theheat-dissipating sheet has insufficient cooling power despite highthermal conductivity. Even if the heat-dissipating sheet were thickerthan 150 μm, further improvement in the cooling power would not beexpected. The preferred thickness of the heat-dissipating sheet is40-100 μm for practical purposes.

(5) Density

The heat-dissipating sheet of the present invention has a density of 2.0g/cm³ or more. Because fine graphite particles have a density of2.25±0.05 g/cm³, the heat-dissipating sheet of the present invention hasa density extremely close to that of fine graphite particles, therebyhaving thermal conductivity close to the inherent thermal conductivityof graphite. The density of the heat-dissipating sheet of the presentinvention is preferably 2.1-2.25 g/cm³.

(6) Thermal Conductivity

As described above, because the heat-dissipating sheet of the presentinvention has a structure in which carbon black composed of channelblack and ketjen black and/or acetylene black is uniformly dispersedamong fine graphite particles, and has a density of 2.0 g/cm³ or more,it has thermal conductivity of 580 W/mK or more in an in-planedirection. The thermal conductivity in an in-plane direction may becalled simply “in-plane thermal conductivity.” The “in-plane direction”is an XY direction in parallel with a surface (XY plane) of theheat-dissipating sheet, and the “thickness direction” is a Z directionperpendicular to the XY plane. The heat-dissipating sheet of the presentinvention preferably has thermal conductivity of 620 W/mK or more in anin-plane direction, and about 10 W/mK or more in a thickness direction.

[2] Production Method of Heat-Dissipating Sheet

(1) Preparation of Dispersion

A dispersion of fine graphite particles, carbon black composed ofchannel black and ketjen black and/or acetylene black, and an organicbinder in an organic solvent is first prepared. Because fine graphiteparticles are easily agglomerated, it is preferable to mix a dispersionof fine graphite particles in an organic solvent with a dispersion ofcarbon black in an organic solvent and a solution of an organic binderin an organic solvent. With the entire dispersion as 100% by mass, thetotal amount of fine graphite particles and carbon black is preferably5-25% by mass. When the total amount of fine graphite particles andcarbon black is less than 5% by mass, too thin a resin-containingcomposite sheet is obtained by one operation. On the other hand, whenthe total amount of fine graphite particles and carbon black is morethan 25% by mass, the concentrations of fine graphite particles andcarbon black are too high in the dispersion, likely causingagglomeration. The more preferred total amount of fine graphiteparticles and carbon black is 8-20% by mass, as long as the mass ratioof fine graphite particles to carbon black is in a range of 75/25 to95/5 as described above.

The mass ratio of the organic binder to the total amount of finegraphite particles and carbon black is preferably 0.01-0.5. When themass ratio of organic binder/(fine graphite particles+carbon black) isless than 0.01, the resultant resin-containing composite sheet is notsufficiently integral, making its handling difficult. When the abovemass ratio is more than 0.5, it takes too much time to burn off theorganic binder in a subsequent burning step, and fine graphite particlesare insufficiently oriented. The mass ratio of organic binder/(finegraphite particles+carbon black) is more preferably 0.02-0.3, mostpreferably 0.03-0.2.

The organic binder used in the present invention is not particularlyrestricted, as long as it can be dissolved in an organic solvent touniformly disperse fine graphite particles and carbon black, and easilyremoved by burning. Such organic binders include, for example, acrylicresins such as polymethylacrylate and polymethylmethacrylate,polystyrenes, polycarbonates, polyvinyl chloride, ABS resins, etc. Amongthem, polymethylmethacrylate and polystyrenes are preferable.

The organic solvent used in the dispersion is preferably an organicsolvent capable of well dispersing fine graphite particles and carbonblack and dissolving an organic binder, and volatile enough to shortenthe drying time. Examples of such organic solvents include ketones suchas methyl ethyl ketone, aliphatic hydrocarbons such as hexane, aromatichydrocarbons such as xylene, alcohols such as isopropyl alcohol, etc.They may be used alone or in combination.

(2) Casting of Dispersion

(a) First Example

FIG. 3 shows a first example of planar die apparatuses for casting thedispersion. This planar die apparatus 10 comprises a lower die plate 11having a flat cavity 11 a extending between opposing sides, and an upperdie plate 12 having a projection 12 a having a complementary shape tothe cavity 11 a. Because the dispersion D used in the present inventionis relatively viscous, it can be cast to a substantially rectangularshape with the cavity 11 a having both open ends as shown in FIG. 4. Thethickness of the cast dispersion D can be made uniform by a doctor blademethod.

(b) Second Example

FIGS. 5(a) and 5(b) show a second example of planar die apparatuses. Alower die plate 21 in this planar die apparatus 20 comprising a lowerdie plate body 22 having a rectangular, flat recess 23, and a platemember 24 placed in the recess 23 on one side. The plate member 24 hasthe same thickness as the depth of the recess 23, and is provided with anotch 24 a on the side of a side surface of the recess 23. As shown inFIG. 5(b), the plate member 24 is placed in the recess 23 of the lowerdie plate body 22 to form a cavity 25. As shown in FIG. 5(b), an upperdie plate 26 has a projection 26 a complementary to the cavity 25. Afterthe dispersion D is cast into the cavity 25 of the lower die plate 21,an excessive dispersion is removed by a doctor blade method to haveuniform thickness as shown in FIG. 6. Because the plate member 24 hasthe notch 24 a, it can be easily taken out of the recess 23, using atool. Accordingly, a composite sheet C of fine graphite particles andcarbon black finally obtained can be easily taken out of the cavity 25by removing the plate member 24.

(c) Third Example

FIGS. 7(a)-7(f) show a third example of planar die apparatuses. Thisplanar die apparatus 30 comprises an upper die plate 31 havingpluralities of flat rectangular projections 31 a on one surface, a flatlower die plate 32, and an intermediate die plate 33 placed on the lowerdie plate 32. The intermediate die plate 33 has pluralities ofrectangular openings 33 a each having a complementary shape to that ofthe rectangular projection 31 a of the upper die plate 31. The lower dieplate 32 has positioning pins 32 a at four corners, and the upper dieplate 31 and the intermediate die plate 33 have holes 31 b, 33 b forreceiving the positioning pins 32 a at four corners. As shown in FIGS.7(e) and 7(f), the pins 32 a of the flat lower die plate 32 are insertedinto the holes 33 b of the intermediate die plate 33, and when theintermediate die plate 33 is placed on an upper surface of the lower dieplate 32, each opening 33 a of the intermediate die plate 33 constitutesa cavity. FIG. 8 shows a dispersion D cast into each cavity formed bythe lower die plate 32 and the intermediate die plate 33, whosethickness is made uniform by removing an excessive dispersion by adoctor blade method. The third example is efficient, because pluralitiesof heat-dissipating sheets are simultaneously formed by one die plate.With each cavity corresponding to a final shape of the heat-dissipatingsheet, the heat-dissipating sheets can be produced efficiently.

(3) Drying of Dispersion

The dispersion D in the cavity of the lower die plate may be driedspontaneously, or by heating to shorten the drying time. The heatingtemperature may be determined depending on the boiling point of anorganic solvent used. For example, when a mixed solvent of xylene andisopropyl alcohol, or methyl ethyl ketone is used, the heatingtemperature is preferably 30-100° C., more preferably 40-80° C. As shownin FIG. 9, the drying of the dispersion D provides a resin-containingcomposite sheet R attached to the cavity 11 a of the lower die plate 11.

(4) Burning

To remove the organic binder, the resin-containing composite sheet R ispreferably burned in a furnace (not shown), without being removed fromthe cavity 11 a of the lower die plate 11. The furnace may be anelectric furnace, a gas furnace, or a continuous furnace in which theresin-containing composite sheet R in the lower die plate 11 is conveyedon a belt conveyor. In the case of a continuous furnace, agradually-cooling furnace is preferably positioned at the end of thecontinuous furnace, to secure gradual cooling described later.

The burning temperature is preferably 550-750° C. When the burningtemperature is lower than 550° C., the removal of the organic bindertakes too much time, and the resultant heat-dissipating sheet cannothave sufficiently high thermal conductivity. On the other hand, when theburning temperature is higher than 750° C., carbon black may be burnedout at least partially, resulting in a heat-dissipating sheet withinsufficient thermal conductivity. The preferred burning temperature is600-700° C.

The resin-containing composite sheet R is burned preferably in anatmosphere sufficiently containing oxygen, for example, in the air. Inan oxygen-containing atmosphere (air), the organic binder is rapidlyburned out without leaving a carbonized binder. However, burning in aninert gas such as a nitrogen gas tends to carbonize the organic binder,providing a heat-dissipating sheet with low thermal conductivity. Theoxygen content in the atmosphere is preferably 10% or more, morepreferably 15% or more.

The burning time of the resin-containing composite sheet R in the abovetemperature range in an oxygen-containing atmosphere is generally 5-30minutes, though variable depending on the burning temperature. Theburning time is a time period in which the resin-containing compositesheet R is kept at the burning temperature, without including thetemperature elevation time and the cooling time. When the burning timeis less than 5 minutes, the organic binder is not completely burned out.When the burning time is more than 30 minutes, carbon black isexcessively exposed to high temperatures, so that carbon black may beburned out at least partially, resulting in a heat-dissipating sheetwith insufficient thermal conductivity. The preferred burning time is7-15 minutes. The temperature elevation time is preferably 10-30minutes.

(5) Cooling

A composite sheet C of fine graphite particles and carbon black formedby burning is preferably gradually cooled in the furnace. It has beenfound that when the composite sheet C of fine graphite particles andcarbon black is left to cool outside the furnace, the resultantheat-dissipating sheet tends to have low thermal conductivity. Thecomposite sheet C of fine graphite particles and carbon black ispreferably gradually cooled over 1 hour or more in the furnace. Thecooling speed is preferably 15° C./minute or less, more preferably 10°C./minute or less. Accordingly, the cooling time is preferably 1 hour ormore.

It has been found that when the composite sheet C of fine graphiteparticles and carbon black is cooled to a temperature equal to or lowerthan the freezing point of water before pressing, the heat-dissipatingsheet exhibits high thermal conductivity in a wide range of the carbonblack content. The cooling temperature may be 0° C. or lower, and ispreferably −5° C. or lower. When cooled to a temperature equal to orlower than the freezing point of water, moisture in the air is likelyfrozen on the composite sheet C. Accordingly, cooling is conductedpreferably in a dry atmosphere. The cooling time is not particularlyrestricted, but may be 10 minutes or more.

(6) Pressing

As shown in FIG. 10, a composite sheet C of fine graphite particles andcarbon black obtained by burning the resin-containing composite sheet Rattached to the cavity 11 a of the lower die plate 11 is pressed bycombining the lower die plate 11 with the upper die plate 12, such thatthe projection 12 a of the upper die plate 12 is pressed onto thecomposite sheet C in the cavity 11 a of the lower die plate 11. Thelower die plate 11 and the upper die plate 12 may be pressed by apressing apparatus, or by a pair of rolls 40, 40 with the compositesheet C sandwiched by the lower die plate 11 and the upper die plate 12as shown in FIG. 11. Pressure applied to the lower die plate 11 and theupper die plate 12 is preferably 20 MPa or more. Pressing is not limitedto once, but may be conducted plural times. Pressing may be conducted atroom temperature, or at high temperature up to 200° C. to increase thepressing efficiency.

During pressing, the lower die plate 11 and the upper die plate 12 arepreferably vibrated via rolls 40. Vibration promotes the densificationof the composite sheet C of fine graphite particles and carbon blackeven under the same pressure. The vibration frequency may be about100-500 Hz. Vibration may be added by a vibration motor.

The heat-dissipating sheet 1 obtained by pressing the composite sheet Cof fine graphite particles and carbon black is peeled from the lower dieplate 11 as shown in FIG. 12. Because of uniform dispersion of carbonblack 3 among fine graphite particles 2 and densification by pressing,the heat-dissipating sheet 1 is neither broken nor cracked when peeledfrom the lower die plate 11. The heat-dissipating sheet 1 thus obtainedhas sufficient bendability, so that it is not broken or fractured evenwhen bent, for example, to 90° with a radius of curvature of 2 cm.

(7) Cutting of Heat-Dissipating Sheet

When a large heat-dissipating sheet 1 is formed by the above process, itshould be cut to a proper size so that it can be attached to a smallelectronic appliance. On the other hand, when a heat-dissipating sheet 1having a use size is formed, its peripheral portion need only betrimmed. Because of uniform dispersion of carbon black 3 among finegraphite particles 2, the heat-dissipating sheet 1 of the presentinvention cut by an ordinary cutter has a sharp cut surface withoutraggedness.

(8) Surface Coating of Heat-Dissipating Sheet

The heat-dissipating sheet 1 of the present invention comprising finegraphite particles and carbon black is preferably coated with aninsulating resin or a plastic film, to prevent the detachment of finegraphite particles and carbon black and to achieve surface insulation.The insulating resins are preferably thermoplastic resins soluble inorganic solvents, for example, acrylic resins such aspolymethylmethacrylate, polystyrenes, polycarbonates, polyvinylchloride, polyurethanes, etc. The insulating plastic films may be madeof polyolefins such as polyethylene and polypropylene, polyesters suchas polyethylene terephthalate, polyamides such as nylons, polyimides,etc. The insulating plastic film preferably has a heat-sealing layer. Aslong as the functions of preventing the detachment of fine graphiteparticles and carbon black and adding insulation are exhibited, thethickness of the insulating resin coating and the insulating plasticfilm may be several micrometers to about 20 μm. Surface coating may bepreferably conducted after cutting the heat-dissipating sheet 1 to adesired size, to surely prevent the detachment of fine graphiteparticles and carbon black from the cut surface of the heat-dissipatingsheet 1.

[3] Heat Dissipation Test

The heat dissipation test of the heat-dissipating sheet of the presentinvention may be conducted by an apparatus 50 shown in FIGS. 13 and 14.This heat dissipation test apparatus 50 comprises a heat-insulating,electric-insulating table 51 having an annular recess 52, a circularplate heater 53 received in the annular recess 52, temperature-measuringthermocouples 54 attached to a lower surface of the heater 53, atemperature controller 55 connected to the heater 53 and thetemperature-measuring thermocouples 54, and a 1-mm-thick acrylic plate(100 mm×100 mm) 57 covering a test piece 56 of 50 mm×100 mm of theheat-dissipating sheet 1 placed on the table 51, at such a position thatthe heater 53 is located at a center of the acrylic plate 57. The testpiece 56 has nine temperature-measuring points t₀-t₈ at positions shownin FIG. 15, a temperature measured at the point t₀ being the highesttemperature (Tmax), an average of temperatures measured at the pointst₁-t₄ being an intermediate temperature (Tm), an average of temperaturesmeasured at the points t₅-t₈ being the lowest temperature (Tmin), and anaverage of Tm and Tmin being an average temperature (Tav).

The present invention will be explained in more detail with Examplesbelow without intention of restricting the present invention thereto.

Example 1

100 parts by mass in total of 85% by mass of fine graphite particles(UP-35N available from Nippon Graphite Industries Ltd., ash: less than1.0%, average size: 25 μm), 10% by mass of channel black (averageprimary particle size: 42 nm) and 5% by mass of ketjen black (EC600JD,average primary particle size: 34 nm, porosity: 80%) were mixed with 10parts by mass of polymethylmethacrylate (PMMA) as an organic binder, and600 parts by mass of a mixed solvent of xylene/isopropyl alcohol (massratio: 6/4) as an organic solvent, to prepare a dispersion of finegraphite particles, carbon black and an organic binder in an organicsolvent (viscosity: 1200 cP). The composition of the dispersioncomprised 12.0% by mass of fine graphite particles, 1.4% by mass ofchannel black, 0.7% by mass of ketjen black, 1.4% by mass of the organicbinder, and 84.5% by mass of the organic solvent.

This dispersion was cast into a 1-mm-deep cavity 11 a of a lower dieplate 11 in the SUS-made planar die apparatus 10 shown in FIG. 3, andmade to have the same thickness as the depth of the cavity 11 a by adoctor blade method. The dispersion D was spontaneously dried for 30minutes to form a resin-containing composite sheet R.

The resin-containing composite sheet R kept in the lower die plate 11was introduced into an electric furnace, and burned at 650° C. for 10minutes in an air atmosphere to remove the organic binder. The resultantcomposite sheet C of fine graphite particles and carbon black wasgradually cooled over about 3 hours in the electric furnace.

The lower die plate 11 containing the composite sheet C of fine graphiteparticles and carbon black in the cavity 11 a was combined with an upperdie plate 12 having a complementary shape to the lower die plate 11,such that a projection 12 a of the upper die plate 12 came into contactwith the composite sheet C of fine graphite particles and carbon black,as shown in FIG. 10, and caused to pass through a gap between a pair ofrolls 40, 40 rotating at a peripheral speed of 30 cm/minute 4 times, topress the composite sheet C of fine graphite particles and carbon blackat linear pressure of 20 MPa or more each time, as shown in FIG. 11.

After pressing, a heat-dissipating sheet 1 could be taken out of thecavity 11 a of the lower die plate 11 without breakage. Theheat-dissipating sheet 1 thus obtained had a thickness of 70 μm and adensity of 2.17 g/cm³. A test piece of 50 mm×100 mm was cut out of thisheat-dissipating sheet 1, and set in the apparatus shown in FIGS. 13-15to conduct a heat dissipation test at room temperature (26.1° C.). Thetest piece was heated at 72° C. (hot spot) by a ceramic heater 53 of2.48 W (50 mm×50 mm) After reaching an equilibrium state, thetemperature at each point of the heat-dissipating sheet test piece wasas follows:

t₀: 48.8° C.,

t₁: 44.7° C.,

t₂: 44.3° C.,

t₃: 44.6° C.,

t₄: 43.8° C.,

t₅: 42.5° C.,

t₆: 42.2° C.,

t₇: 39.8° C., and

t₈: 39.9° C.

Thus, the highest temperature Tmax was 48.8° C. (hot spot), theintermediate temperature Tm was (44.7° C.+44.3° C.+44.6° C.+43.8°C.)/4=44.4° C., the lowest temperature Tmin was (42.5° C.+42.2° C.+39.8°C.+39.9° C.)/4=41.1° C., and the average temperature Tav was(Tm+Tmin)/2=42.8° C.

The thermal conductivity (W/mK) of the heat-dissipating sheet 1 wascalculated as a product of thermal diffusivity (m²/s) measured by alaser flash method and heat capacity (density×specific heat). Thespecific heat was regarded as 750. As a result, the thermal conductivityof the heat-dissipating sheet 1 was 660 W/mK in an in-plane directionand 10 W/mK in a thickness direction.

When this heat-dissipating sheet 1 was bent to 90° with a radius ofcurvature of 2 cm, no breakage occurred. The heat-dissipating sheet 1cut by scissors had a clear-cut surface with no fine graphite particlesand carbon black detached.

Example 2

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for using channel black having an average primary particle sizeof 85 nm, and ketjen black (EC300J) having an average primary particlesize of 40 nm and porosity of 60%, as carbon black. The heat-dissipatingsheet had an in-plane thermal conductivity of 600 W/mK. When thisheat-dissipating sheet 1 was bent to 90° with a radius of curvature of 2cm, no breakage occurred. The heat-dissipating sheet 1 cut by scissorshad a clear-cut surface with no fine graphite particles and carbon blackdetached.

Example 3

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for using channel black having an average primary particle sizeof 85 nm in place of channel black having an average primary particlesize of 42 nm. The heat-dissipating sheet 1 had an in-plane thermalconductivity of 610 W/mK. When this heat-dissipating sheet 1 was bent to90° with a radius of curvature of 2 cm, no breakage occurred. Theheat-dissipating sheet 1 cut by scissors had a clear-cut surface with nofine graphite particles and carbon black detached.

Example 4

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for changing the amount of ketjen black to 3% by mass. Theheat-dissipating sheet 1 had an in-plane thermal conductivity of 640W/mK. When this heat-dissipating sheet 1 was bent to 90° with a radiusof curvature of 2 cm, no breakage occurred. The heat-dissipating sheet 1cut by scissors had a nearly clear-cut surface, which was poorer thanthat of Example 1, with substantially no fine graphite particles andcarbon black detached.

Example 5

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for changing the amount of channel black to 15% by mass. Theheat-dissipating sheet 1 had an in-plane thermal conductivity of 590W/mK. When this heat-dissipating sheet 1 was bent to 90° with a radiusof curvature of 2 cm, no breakage occurred. The heat-dissipating sheet 1cut by scissors had a clear-cut surface with no fine graphite particlesand carbon black detached.

Example 6

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for changing the amount of ketjen black to 10% by mass. Theheat-dissipating sheet 1 had an in-plane thermal conductivity of 580W/mK. When this heat-dissipating sheet 1 was bent to 90° with a radiusof curvature of 2 cm, no breakage occurred. The heat-dissipating sheet 1cut by scissors had a clear-cut surface with no fine graphite particlesand carbon black detached.

Example 7

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for changing the amount of channel black to 5% by mass and theamount of ketjen black to 10% by mass. The heat-dissipating sheet 1 hadan in-plane thermal conductivity of 630 W/mK. When this heat-dissipatingsheet 1 was bent to 90° with a radius of curvature of 2 cm, no breakageoccurred. The heat-dissipating sheet 1 cut by scissors had a clear-cutsurface with no fine graphite particles and carbon black detached.

Example 8

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for changing the amount of channel black to 5% by mass and theamount of ketjen black to 3% by mass. The heat-dissipating sheet 1 hadan in-plane thermal conductivity of 650 W/mK. When this heat-dissipatingsheet 1 was bent to 90° with a radius of curvature of 2 cm, no breakageoccurred. The heat-dissipating sheet 1 cut by scissors had a nearlyclear-cut surface, which was poorer than that of Example 1, withsubstantially no fine graphite particles and carbon black detached.

Example 9

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for using acetylene black having an average primary particlesize of 48 nm in place of ketjen black. The heat-dissipating sheet 1 hadan in-plane thermal conductivity of 630 W/mK. When this heat-dissipatingsheet 1 was bent to 90° with a radius of curvature of 2 cm, no breakageoccurred. The heat-dissipating sheet 1 cut by scissors had a nearlyclear-cut surface, which was poorer than that of Example 1, withsubstantially no fine graphite particles and carbon black detached.

Comparative Example 1

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for using only 15% by mass of channel black having an averageprimary particle size of 42 nm as carbon black. The heat-dissipatingsheet 1 had an in-plane thermal conductivity of 640 W/mK. However, whenthis heat-dissipating sheet 1 was bent to 90° with a radius of curvatureof 2 cm, breakage sometimes occurred.

Comparative Example 2

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for using only 15% by mass of ketjen black having an averageprimary particle size of 34 nm as carbon black. The heat-dissipatingsheet 1 had an in-plane thermal conductivity of 610 W/mK. Theheat-dissipating sheet 1 did not have strength necessary for handling,though no breakage occurred when it was bent to 90° with a radius ofcurvature of 2 cm.

Comparative Example 3

A heat-dissipating sheet 1 was produced in the same manner as in Example1 except for changing the amount of channel black to 20% by mass and theamount of ketjen black to 10% by mass. The heat-dissipating sheet 1 hadas low in-plane thermal conductivity as 550 W/mK.

With respect to the heat-dissipating sheets of Examples 1-9 andComparative Examples 1-3, the compositions and thermal conductivitiesare shown in Table 1.

TABLE 1 Composition (% by mass) Carbon Black CB⁽¹⁾ KB⁽²⁾ AB⁽³⁾ ThermalGraphite 42 85 34 40 48 Conductivity No. 25 μm nm nm nm nm nm (W/mK)Example 1 85 10 — 5 — — 660 Example 2 85 — 10 — 5 — 600 Example 3 85 —10 5 — — 610 Example 4 87 10 — 3 — — 640 Example 5 80 15 — 5 — — 590Example 6 80 10 — 10 — — 580 Example 7 85 5 — 10 — — 630 Example 8 92 5— 3 — — 650 Example 9 85 10 — — — 5 630 Com. Ex. 1 85 15 — — — — 640Com. Ex. 2 85 — — 15 — — 610 Com. Ex. 3 70 20 — 10 — — 550 Note:⁽¹⁾Channel black. ⁽²⁾Ketjen black. ⁽³⁾Acetylene black.

Conventional Example 1

The same heat dissipation test as in Example 1 was conducted on agraphite sheet PGS as thick as 70 μm (available from PanasonicCorporation). As a result, the temperature at each point of a test pieceof the heat-dissipating sheet was as follows:

t₀: 48.4° C.,

t₁: 44.3° C.,

t₂: 43.9° C.,

t₃: 44.2° C.,

t₄: 43.4° C.,

t₅: 40.1° C.,

t₆: 38.0° C.,

t₇: 38.7° C., and

t₈: 36.8° C.

Thus, the highest temperature Tmax was 48.4° C. (hot spot), theintermediate temperature Tm was (44.3° C.+43.9° C.+44.2° C.+43.4°C.)/4=44.0° C., the lowest temperature Tmin was (40.1° C.+38.0° C.+38.7°C.+36.8° C.)/4=38.4° C., and the average temperature Tav was(Tm+Tmin)/2=41.2° C. Comparison with Example 1 revealed that thegraphite sheet of Conventional Example 1 was poorer than that of Example1 in any of the highest temperature Tmax, the lowest temperature Tminand the average temperature Tav.

Example 10

A heat-dissipating sheet 1 was produced in the same manner as in Example1, except for changing the ratio (% by mass) of channel black in thecarbon black. FIG. 16 shows the relation between thermal conductivityand the ratio of channel black in the heat-dissipating sheet 1. It isclear from FIG. 16 that a higher ratio of channel black provides higherthermal conductivity. However, when the carbon black was composed onlyof channel black, the heat-dissipating sheet 1 was broken when bent to90° with a radius of curvature of 2 cm. This indicates that the carbonblack should be composed of not only channel black but also ketjen black(and/or acetylene black).

Example 11

A heat-dissipating sheet 1 was produced in the same manner as in Example1, except that freezing was conducted at −5° C. for 30 minutes afterburning the resin-containing composite sheet R. The resultantheat-dissipating sheet 1 had higher in-plane thermal conductivity thanthat of Example 1. When the heat-dissipating sheet 1 of Example 11 wasbent to 90° with a radius of curvature of 2 cm, no breakage occurred asin Example 1. As in Example 1, the heat-dissipating sheet 1 cut byscissors had a clear-cut surface, with no fine graphite particles andcarbon black detached.

Effects of the Invention

Because the heat-dissipating sheet of the present invention has astructure in which carbon black is uniformly dispersed among finegraphite particles, a mass ratio of fine graphite particles to carbonblack being 75/25 to 95/5, and the carbon black being composed ofchannel black and ketjen black and/or acetylene black, it has a densityof 2.0 g/cm³ or more and an in-plane thermal conductivity of 580 W/mK ormore. Also, because carbon black composed of fine channel black and fineketjen black and/or acetylene black is uniformly dispersed among finegraphite particles, the heat-dissipating sheet of the present inventionhas uniform thermal conductivity as well as sufficient mechanicalproperties for handling. Such a uniform, high-density heat-dissipatingsheet is obtained by forming a resin-containing composite sheetcomprising uniformly dispersed fine graphite particles and carbon blackfrom a dispersion comprising fine graphite particles, carbon black andan organic binder, burning the resin-containing composite sheet toremove the organic binder, and then pressing it for densification.

Because the heat-dissipating sheet of the present invention is producedby a low-cost process of applying, burning and pressing a relativelyinexpensive material comprising fine graphite particles and carbonblack, it is advantageously inexpensive, with as high in-plane thermalconductivity as 580 W/mK or more and sufficient mechanical propertiesfor handling. The heat-dissipating sheet of the present invention havingsuch feature is suitable for small electronic appliances such asnote-type personal computers, smartphones, mobile phones, etc.

The invention claimed is:
 1. A method for producing a heat-dissipatingsheet comprising the steps of (1) preparing a dispersion of finegraphite particles, carbon black and an organic binder in an organicsolvent, a mass ratio of said fine graphite particles to said carbonblack being 75/25 to 95/5, and said carbon black being composed ofchannel black and ketjen black and/or acetylene black; (2) casting saiddispersion into a cavity of a lower die plate and then drying it to forma resin-containing composite sheet comprising said fine graphiteparticles, said carbon black and said organic binder; (3) burning saidresin-containing composite sheet to remove said organic binder to form acomposite sheet of fine graphite particles and carbon black; and (4)pressing said lower die plate combined with an upper die plate todensify said composite sheet of fine graphite particles and carbonblack.
 2. The method for producing a heat-dissipating sheet according toclaim 1, wherein said dispersion comprises 5-25% by mass in total offine graphite particles and carbon black, and 0.5-2.5% by mass of theorganic binder.
 3. The method for producing a heat-dissipating sheetaccording to claim 1, wherein a mass ratio of said organic binder to thetotal amount of said fine graphite particles and said carbon black is0.01-0.5.
 4. The method for producing a heat-dissipating sheet accordingto claim 1, wherein said organic binder is an acrylic resin, apolystyrene resin or polyvinyl alcohol.
 5. The method for producing aheat-dissipating sheet according to claim 1, wherein said organicsolvent is at least one selected from the group consisting of ketones,aromatic hydrocarbons and alcohols.
 6. The method for producing aheat-dissipating sheet according to claim 1, wherein said burning stepis conducted at a temperature of 550-700° C.
 7. The method for producinga heat-dissipating sheet according to claim 1, wherein cooling to roomtemperature after burning is gradually conducted over 1 hour or more. 8.The method for producing a heat-dissipating sheet according to claim 1,wherein said pressing step is conducted, after said composite sheet offine graphite particles and carbon black is cooled to a temperatureequal to or lower than the freezing point of water.
 9. The method forproducing a heat-dissipating sheet according to claim 1, wherein saidpressing step is conducted at pressure of 20 MPa or more.
 10. The methodfor producing a heat-dissipating sheet according to claim 1, whereinsaid resin-containing composite sheet formed in said lower die platecavity is burned without being peeled from said lower die plate, andthen pressed with said lower die plate combined with said upper dieplate.
 11. The method for producing a heat-dissipating sheet accordingto claim 1, wherein said pressing step is conducted at a temperature ina range of room temperature to 200° C.